Hi Adandrea,
just taking a quick look it looks as if you just want to remove the html tags and the best way I would use is to do something similar to what you did with Regex because using HTMLAgaility pack is mainly for pulling apart a page and pulling specific information.
for example a table always appears on a page named id="lookingforme" and you can pull the details out of that table and use them I can give you a sample for this. But if you give me a sample of the html you want to strip out the tags from but leave the italic
and bold tags I could see what I can do.
Regards,
Lee